In economic terms, a public good is not one that is necessarily good for the public. In other words, this is not a question about whether TV is a good thing. Instead, a public good is one that has two characteristics that separate it from most other goods. First, it has “nonrival consumption.” This means that, when one person consumes it, there is still just as much of it left for the next person. Each person’s use does not diminish the amount of it that is available. Second, it is a good where nonpayers cannot be excluded from using it. In other words, there is no way to make people pay for it and to prevent those who do not pay from using it.
Broadcast television fits both of these criteria. Once a signal is broadcast, any number of people can use it. Within the range of the signal, there is no limit on how many people can use it because the amount of broadcast TV does not diminish at all when one TV set captures the broadcast. This means it has nonrival consumption. There is also no way to prevent people from getting broadcast TV. As long as you have an antenna, you can receive the signal.
For these reasons, economists say that broadcast television is a public good.